The present invention relates to a liquid crystal display device and a fabrication method for the same, and more particularly, to a liquid crystal display device suitably used for portable information terminals (for example, PDAs), mobile phones, car-mounted liquid crystal displays, digital cameras, PCs, amusement equipment, TVs and the like.
The information infrastructure is advancing day to day, and equipment such as mobile phones, PDAs, digital cameras, video cameras and car navigators has penetrated deeply into people's lives. Liquid crystal display (LCD) devices have been adopted in most of such equipment. With increase of the information amount handled with the bodies of the equipment, LCD devices are requested to display a larger amount of information, and are demanded by the market for higher contrast, a wider viewing angle, higher brightness, multiple colors and higher definition.
A vertical alignment mode using a vertically aligned liquid crystal layer has increasingly received attention as a display mode enabling high contrast and a wide viewing angle. A vertically aligned liquid crystal layer is generally obtained using a vertical alignment film and a liquid crystal material having negative dielectric anisotropy.
For example, Japanese Laid-Open Patent Publication No. 6-301036 (Literature 1) discloses an LCD device in which a tilt electric field is generated around an opening formed in a counter electrode that faces a pixel electrode via a liquid crystal layer, so that liquid crystal molecules surrounding liquid crystal molecules existing in the opening, which are in the vertically aligned state, are aligned in tilt directions around the opening as the center, to thereby improve the visual angle characteristics.
However, in the device described in Literature 1, it is difficult to generate a tilt electric field over the entire region of each pixel. Therefore, each pixel has a region in which liquid crystal molecules delay in response to a voltage, and this causes a problem of occurrence of an afterimage phenomenon.
Japanese Laid-Open Patent Publication No. 2002-55347 (Literature 2) discloses a technology in which slit electrodes (an opening pattern) are provided in both pixel electrodes and a counter common electrode and, in at least either the pixel electrodes or the common electrode, steps are provided in regions having the slit electrodes to allow an electric field to tilt in four directions in uniform distribution using the opening pattern, to thereby attain a wide viewing angle.
Japanese Laid-Open Patent Publication No. 2003-167253 (Literature 3) discloses a technology in which a plurality of projections are provided regularly in each pixel to stabilize the aligned state of liquid crystal domains having radially inclined alignment formed around the projections. This literature also discloses using a tilt electric field generated at openings formed in an electrode, together with the alignment regulating force of the projections, to regulate the alignment of liquid crystal molecules, and thus improve the display characteristics.
In recent years, a type of LCD device providing high-quality display both outdoors and indoors has been proposed (see Japanese Patent Gazette No. 2955277 (Literature 4) and U.S. Pat. No. 6,195,140 (Literature 5), for example). In this type of LCD device, called a transflective LCD device, each pixel has a reflection region in which display is done in the reflection mode and a transmission region in which display is done in the transmission mode.
The currently available transflective LCD devices adopt an ECB mode, a TN mode and the like. Literature 3 described above discloses adoption of the vertical alignment mode for a transflective LCD device, not only for a transmissive LCD device. Japanese Laid-Open Patent Publication No. 2002-350853 (Literature 6) discloses a technology in which in a transflective LCD device having a vertically aligned liquid crystal layer, the alignment (multi-axis alignment) of liquid crystal molecules is controlled with depressions formed on an insulating layer that is provided to double the thickness of the liquid crystal layer in a transmission region compared with that in a reflection region. According to this literature, the depressions are in the shape of a regular octagon, for example, and projections or slits (electrode openings) are formed at positions opposing the depressions via the liquid crystal layer (see FIGS. 3 and 16 of Literature 6, for example).
To improve the display quality in the reflection mode, a technique of forming a diffuse reflection layer excellent in diffuse reflection characteristics has been examined. For example, Japanese Laid-Open Patent Publication No. 6-75238 (Literature 7) discloses a technique in which fine projections and depressions are formed randomly on the surface of a reflection electrode in a photolithography step using a two-layer photosensitive resin film to thereby obtain good diffuse reflection characteristics. Japanese Laid-Open Patent Publication No. 9-90426 (Literature 8) discloses a technique in which a reflection electrode having fine projections and depressions is formed by exposing a one-layer photosensitive resin film to light via a photomask for formation of contact holes and fine projections and depressions and developing the resin, for simplification of the fabrication process.
The technology disclosed in Literature 3 has the following problems. Projections are provided in each pixel to form a plurality of liquid crystal domains in the pixel (that is, divide the pixel into domains), to thereby strengthen the alignment regulating force on liquid crystal molecules. According to examinations conducted by the inventors of the present invention, however, to obtain sufficient alignment regulating force, an alignment control structure made of projections regularly placed inside each pixel must be formed, and this complicates the fabrication process. The contrast ratio may decrease due to light leakage occurring in the peripheries of the projections in the pixel. If a light-shading portion is provided to prevent decrease in contrast ratio, the effective aperture ratio may possibly decrease.
In the technology disclosed in Literature 6, it is necessary to provide projections or electrode openings at positions opposite to the depressions formed for control of the multi-axis alignment. This technology therefore has the same problems as those described above.
In view of the above, an object of the present invention is providing a liquid crystal display device having at least one axisymmetrically aligned domain (also called a “radially inclined alignment domain”) in each pixel, which can sufficiently stabilize the alignment of liquid crystal molecules and suppress decrease in contrast ratio or effective aperture ratio.
The fabrication process will be complicated if the methods disclosed in Literature 7 and 8 are employed to form reflection electrodes to improve the display quality in the reflection mode of a transflective LCD device, for example. That is, it is necessary to form fine projections and depressions for improving the diffuse reflection characteristics, in addition to the projections for alignment regulation, and this increases the cost of the LCD device.
In view of the above, another object of the present invention is providing a liquid crystal display device that can sufficiently stabilize the alignment of liquid crystal molecules with a comparatively simple construction and for which the fabrication process is simple compared with the conventional ones, and a fabrication method for such a liquid crystal display device.
Yet another object of the present invention is providing a liquid crystal display device having at least one axisymmetrically aligned domain in each pixel, which can sufficiently stabilize the alignment of liquid crystal molecules with a comparatively simple construction having an alignment regulating structure for the axisymmetric alignment only on one substrate of the device, to provide display quality equal to or higher than that conventionally obtained.